This invention relates generally to insulator layers for gallium arsenide semiconductor devices and, more particularly, to insulator layers derived from Group IIa metal fluorides which are formed on single crystal gallium arsenide substrates.
Materials used in semiconductor devices must have good semiconducting properties, good electron mobility, and the ability to host an insulating material. Several materials are available which have good semiconducting properties and good electron mobilities but which are unsuitable because a good insulator cannot be formed on them. Silicon, however, is widely used in semiconductor devices because silicon dioxide forms naturally on silicon and silicon dioxide is a good insulator. The disadvantage of silicon is that its mobility is not as high as other semiconductors and silicon dioxide is not the strongest insulator available. This means that compromises in speed and performance are made when silicon is used in electronic devices.
Gallium arsenide (GaAs) is also a semiconductor and is used in some electronic applications. A device made out of GaAs would be faster than the same device made out of silicon because GaAs has an electron mobility that is considerably higher than that of silicon. Unfortunately, there is no native insulating oxide suitable for GaAs electronic devices. Also, many opto-electronic devices using GaAs substrates rely on epitaxial insulator/semiconductor heterostructures with abrupt interfaces which further increases the challenge of finding a suitable insulator for GaAs substrates for these applications.
Several materials have been used to provide insulating films on III-V compound semiconductor devices. Some of these films were previously used on silicon semiconductor devices. Examples of these film materials include SiO.sub.2, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, and P.sub.2 O.sub.3 films. New films have also been developed specifically for the III-V compound semiconductors. For instance, A. J. Shuskus (U.S. Pat. No. 4,546,372) developed an essentially oxygen-free, amorphous, phosphorous-nitrogen glass passivating film for III-V compound semiconductors. Similarly, J. Nishizawa et al. (U.S. Pat. No. 4,436,770) disclose new gallium oxynitride and aluminum oxynitride insulating films for III-V compound semiconductors. However, these materials have found only limited application.
Although the high dielectric strength of barium fluoride (BaF.sub.2) (100) makes it a potentially interesting candidate as an insulating material, unfortunately BaF.sub.2 and GaAs are severely lattice mismatched (.about.10% mismatched), which is a condition previously considered to be detrimental to epitaxial growth. The lattice mismatch problem has not been the only condition hindering the growth of epitaxial BaF.sub.2 (100) on GaAs. It is known that the (100) face of the cubic fluorite structure of barium fluoride has a surface free energy far in excess of the (111) face. See, L. J. Schowalter et al., CRC Critical reviews in Solid State and Materials Sciences, pp. 367 (1989). For this reason, it has been thought that (111) growth is favored over (100) growth. Even when (100) growth is achieved, surface free energy considerations predict faceting of the (100) growth front into (111) asperites. This behavior has been observed for CaF.sub.2, as described in the above-cited Schowalter et al. publication, and BaF.sub.2, as described by M. F. Stumborg et al., J. Appl. Phys. 77(6), 2739 (1995).
Clemens el al., J. Appl. Phys. 66(4), 1680 (1989), reported being able to grow (100)-oriented BaF.sub.2 on GaAs, but the orientation switched to (111) after only .about.20 .ANG. of film thickness. Furthermore, streaky RHEED patterns were not observed until the (111) orientations began to dominate. Films grown with a 400.degree. C. substrate temperature exhibited signs of misoriented mosaic structures (i.e., rings in the RHEED patterns). Depositions at higher temperatures (580.degree. C.) did not yield streaky RHEED patterns, leading Clemens el al. to state a conclusion that no two-dimensional growth of (100)-oriented barium fluoride seemed possible.
Truscott et al., J of Crystal Growth, 81, 552 (1987), also investigated the growth of BaF.sub.2 on GaAs as well as the reverse heterostructure. They also found temperature dependent RHEED patterns indicative of (100) and (111) growth modes. However, they reported no achievement of two-dimensional BaF.sub.2 (100) layers. In fact, they reported their BaF.sub.2 films to be conducting, presumably due to Ga diffusion into the BaF.sub.2 layer.
In view of the above, it would be desirable to provide an improved thin film insulator for gallium arsenide electronic devices which yields high quality device characteristics such as high-break down voltage.